Method of forming a part with a globular microstructure

ABSTRACT

A method forms a die-cast metal part having a globular microstructure. The method includes the steps of introducing a metal charge into a chamber in a molten state above the liquidus temperature, and holding the metal charge in the chamber without stirring or agitation until it attains a solid fraction state with a non-homogenous, substantially dendritic microstructure. The metal charge is forced from the chamber through an orifice such that passage through the orifice imparts sheer that breaks the substantially dendritic microstructure to a substantially globular non-dendritic microstructure. The metal charge with the substantially globular non-dendritic microstructure is forced into a cavity having the shape of the die-cast part.

RELATED APPLICATIONS

This application claims the benefit of provisional U.S. Application Ser. No. 60/587215, filed on Jul. 12, 2004, entitled “Method of Forming a Globular Structure,” which is incorporated by reference.

TECHNICAL FIELD

This technology relates to the formation of die-cast metal parts.

BACKGROUND

In the formation of die-cast metal parts, it is preferable for the metal to have a globular microstructure rather than a dendritic microstructure. A globular microstructure is typically imparted to the molten metal material by agitation when the material is cooling between the solidus and liquidus temperatures. Such agitation breaks dendrites into nuclei from which a globular microstructure develops upon further cooling of the material in the die-casting process.

SUMMARY

A method forms a die-cast metal part having a globular microstructure. The method includes the steps of introducing a metal charge into a chamber in a molten state above the liquidus temperature, and holding the metal charge in the chamber without stirring or agitation until it attains a solid fraction state with a non-homogenous, substantially dendritic microstructure. The metal charge is forced from the chamber through an orifice such that passage through the orifice imparts sheer that breaks the substantially dendritic microstructure to a substantially globular non-dendritic microstructure. The metal charge with the substantially globular non-dendritic microstructure is forced into a cavity having the shape of the die-cast part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating an example of the method of forming a globular structure.

FIGS. 2-4 are schematic illustrations of an apparatus used to perform the method.

DESCRIPTION

In step 1, a non-ferrous alloy or metal, such as aluminum, is introduced into a chamber. The chamber may, for example, be a shot sleeve or some other type of insulated or controlled-heating chamber. The liquid non-ferrous alloy or metal may be introduced into the chamber using gravity, a pump or some other known means. Step 1 is further illustrated in FIG. 2.

FIG. 2 shows an example chamber 10 that has been filled with a liquid non-ferrous alloy or metal 16. The metal is controlled above the solidus temperature but is cooled to below it's liquidus temperature. The chamber 10 includes an orifice 12 and a piston 14 for forcing the non-ferrous alloy or metal 16 out of the chamber 10 through the orifice 12. It should be understood that other means for forcing the non-ferrous alloy or metal 16 out of the orifice 12 may also be used. That is, the piston 14 may be replaced with other mechanical, hydraulic, magnetic or other device for forcing the non-ferrous alloy or metal 16 from the chamber 10 through the orifice 12.

With reference again to FIG. 1, in step 2 of the example method the non-ferrous alloy or metal is held in the chamber to extract a pre-determined (X) amount of enthalpy by exchange. It is intended that the overall energy in the system for the metal, upon equilibration, remains above the solidus. During step 2, the solid fraction of the non-ferrous alloy or metal increases. For example, the alloy or metal may form a dendritic structure. In addition, the resulting alloy or metal structure may not be homogeneous. For example, in addition to a dendritic structure, some solid or partial-solid material (i.e., non-ferrous alloy or metal with a higher solid fraction) may form within the chamber during step 2. Step 2 is further illustrated in FIG. 3. The material in the chamber 10 preferably attains a solid fraction state rather than a semi-solid state. By “solid fraction state” it is meant that the material is between the liquidus and solidus temperatures, but is either a) 1-49% liquid and 51-99% solid, or b) 1-49% solid and 51-99% liquid, whereas a material in a semi-solid state is 50% solid and 50% liquid. FIG. 3 illustrates a dendritic structure 20 and a formed solid or partial-solid structure 22 within the chamber 10.

In step 3, the non-ferrous alloy or metal that contains a portion of solid fraction as dendrites is forced from the chamber through the chamber orifice 12 to create a globular structure 32. The chamber orifice 12 includes edges 34 or other structures sufficient to shear the alloy or metal passing through the orifice 12 or otherwise break or create work to sufficiently break the dendrite arms of the alloy/metal. The resulting globular structure 32 may then be used to fill a mold 30 in step 4.

Steps 3 and 4 of the example method are further illustrated by FIG. 4. FIG. 4 shows an example chamber 10 in which the dendritic structure 20 has been forced through the chamber orifice 12 to create a globular structure 32. As illustrated, the chamber orifice 12 includes edges 34 to shear/break the dendrite arms as the alloy/metal passes through the orifice. Also illustrated is a mold 30 for forming the globular structure 32 into a solid cast structure.

This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art. 

1. A method of forming a die-cast metal part having a globular microstructure, comprising: introducing a metal charge into a chamber in a molten state above the liquidus temperature; holding the metal charge in the chamber without stirring or agitation until the metal charge attains a solid fraction state with a non-homogenous, substantially dendritic microstructure; forcing the metal charge having the solid fraction state with the non-homogenous, substantially dendritic microstructure from the chamber through an orifice such that passage of the metal charge through the orifice imparts shear that breaks the substantially dendritic microstructure to a substantially globular non-dendritic microstructure; and forcing the metal charge with the substantially globular non-dendritic microstructure into a cavity having the shape of the die-cast part.
 2. A method as defined in claim 1 wherein the metal is aluminum. 